1. Field of the Invention
This invention relates to a surface treatment method for aluminum or an aluminum alloy, and pistons surface-treated thereby, as well as a surface-treating film for aluminum or an aluminum alloy, and sliding members having a sliding surface coated therewith.
More particularly, this invention relates to a surface treatment method which requires simple equipment, can reduce treatment costs, and can yield aluminum or an aluminum alloy having excellent abrasion resistance, corrosion resistance and other properties, as well as pistons having undergone a surface treatment according to this method. It also relates to a surface-treating film suitable for use on the sliding surfaces of internal combustion engines and having excellent abrasion resistance, initial fitness, oil retention and other properties, and sliding members coated with such a slide film.
2. Description of the Related Art
The Alumite treatment which has conventionally been employed is a method for anodizing aluminum in an acid bath to form a hard aluminum oxide film on the aluminum surface. However, this method has the disadvantage that it requires equipment for electric power supply and that it involves a considerable cost because of a slow rate of film formation.
On the other hand, the skirt of an aluminum or aluminum alloy piston as an internal combustion engine component is plated with tin. Although the deposited tin film is effective in bring about good initial fitness, it cannot be expected to have the effect of improving abrasion resistance.
In view of the above-described problems of conventional surface treatment techniques, the present inventors made intensive investigations for the purpose of developing a surface treatment method which requires simple equipment, can reduce treatment costs, and can form a uniform film having excellent corrosion resistance, abrasion resistance and other properties, as well as sliding members surface-treated by such a method.
As a result, the present inventors have found that the above-described problems can be solved by a surface treatment method comprising the steps of providing a treating solution (or aqueous solution) containing magnesium silicofluoride (MgSiF6xc2x76H2O) and a specific ammonium compound or the like, heating this treating solution to a temperature in the range of 70 to 100xc2x0 C., and soaking aluminum or an aluminum alloy in this treating solution.
Moreover, it has been found that the above-described problems can also be solved by coating the whole surface of a sliding member or the sliding surface thereof with a specific film consisting of a mixture of NH4MgAlF6 and MgAlF5xc2x71.5H2O or a mixture of NH4MgAlF6 and MgAl2F8xc2x72H2O.
Furthermore, it has been found that the above-described problems can also be solved, for example, by using a specific film formed on a surface of aluminum or an aluminum alloy, consisting of a mixture of NH4MgAlF6 and MgAlF5xc2x71.5H2O or a mixture of NH4MgAlF6 and MgAl2F8xc2x72H2O, and having silicon particles dispersed therein. The present invention has been completed from this point of view.
That is, according to a first aspect of the present invention, there is provided a surface treatment method comprising the steps of providing a treating solution containing an ammonium compound selected from ammonium borofluoride and ammonium chloride, or aqueous ammonia, and magnesium silicofluoride (MgSiF6xc2x76H2O); heating the treating solution to a temperature of 70 to 100xc2x0 C.; and soaking aluminum or an aluminum alloy in the treating solution. In this surface treatment method, the aforesaid treating solution preferably contains 0.1 to 20 parts by weight of magnesium silicofluoride and 0.01 to 10 parts by weight, as expressed in terms of ammonium (NH4), of the ammonium compound or aqueous ammonia. The ammonium compound used herein preferably comprises a compound which has a solubility of not less than 1 g/L in water and, when dissolved, can provide ammonium ions (NH4+) to the solution. Specifically, ammonium borofluoride (NH4BF4) or ammonium chloride (NH4Cl) is used.
Moreover, the present invention also provides a piston having undergone a surface treatment according to the above-described surface treatment method.
According to a second aspect of the present invention, there is provided a piston wherein a surface thereof is coated with a film consisting of a mixture of NH4MgAlF6 and MgAlF5xc2x71.5H2O or a mixture of NH4MgAlF6 and MgAl2F8xc2x72H2O, and preferably its whole surface including the piston ring grooves, piston pin boss, skirt, piston head and internal piston surface is coated with the film. In this piston, the thickness of the film is preferably in the range of 1 to 20 xcexcm.
According to a third aspect of the present invention, there is provided a sliding member made of a base metal comprising aluminum or an aluminum alloy, wherein the whole surface of the sliding member or the sliding surface thereof is coated with a film which consists of a mixture of NH4MgAlF6 and MgAlF5xc2x71.5H2O or a mixture of NH4MgAlF6 and MgAl2F8xc2x72H2O, has a cubic crystal structure, and shows no crystalline orientation.
Moreover, there is also provided a sliding member wherein the whole surface of the sliding member or the sliding surface thereof is coated with a film which has a thickness of 1 to 20 xcexcm and consists of a plurality of aggregates having a size of 1 to 20 xcexcm, each aggregate being formed of microcrystals having a size of 1 xcexcm or less.
According to a fourth aspect of the present invention, there is provided a surface-treating film for aluminum alloys wherein the surface-treating film is a film formed on a surface of aluminum or an aluminum alloy, consisting of a mixture of NH4MgAlF6 and MgAlF5xc2x71.5H2O or a mixture of NH4MgAlF6 and MgAl2F8xc2x72H2O, and having silicon particles dispersed therein, the content of silicon particles dispersed in the film is in the range of 1 to 24% by weight and preferably 6 to 24% by weight, and the content of silicon in the aluminum alloy is in the range of 4 to 24% by weight and 7 to 24% by weight.